1. Field of the Invention
The present invention relates generally to exterior wall systems for commercial and residential structures. In another aspect, the invention concerns lightweight precast concrete wall panels.
2. Description of the Prior Art
Precast concrete wall panels have been used for years to provide durable and aesthetically pleasing exterior walls. One disadvantage of traditional precast concrete wall panels is the weight of the panels. The high weight of conventional precast wall panels can make them expensive to ship and erect. Further, because heavy wall panels cause deflection of structural steel wall members supporting the panels, the strength of the steel frame of a building may need to be increased in order to adequately support concrete wall panels without excessive deflection. Such a need to increase the strength of the structural steel members of a building can add significantly to the overall cost of the building.
In recent years, several lightweight alternatives to traditional precast concrete wall panels have been used. One such system is commonly known as EIFS (Exterior Insulation and Finish System). EIFS is a multi-layered exterior wall system that typically consists of a lightweight pliable insulation board covered with a fiberglass reinforced base coat that is coated with a colored acrylic finish coat. Although EIFS is lightweight and provides thermal insulation, a number of drawbacks are associated with EIFS. For example, EIFS walls have a tendency to crack and allow moisture to seep between the EIFS layers or between the innermost EIFS layer and the interior wall. In either case, such leakage can cause water damage and/or damage due to mold or mildew. In fact, the tendency of EIFS wall systems to leak has caused many insurance companies to stop writing policies covering EIFS structures. A further disadvantage of EIFS is its lack of durability. For example, simply bumping an EIFS wall with a lawn mower or other equipment during routine lawn maintenance can physically and visibly damage the EIFS wall, thereby necessitating expensive repair. Another problem with EIFS is the inability to form a true caulk joint at the edge of the wall. This inability to form a true caulk joint is caused by the fact that EIFS walls lack a sufficiently thick rigid edge. A proper caulk joint typically requires at least one inch of rigid edge so that a backer-rod can be inserted into a joint and a bead of caulk can fill the joint and seal against at least one half inch of the rigid edge. This allows the seal to maintain integrity during normal shifting and expansion/contraction of the structure. Thus, the lack of a true caulk joint in EIFS walls can contribute to moisture leakage.
Another lightweight wall system that has been introduced in recent years employs precast GFRC (Glass Fiber Reinforced Concrete) wall panels. GFRC wall panels are relatively strong compared to EIFS, but have a number of drawbacks. The main drawback of GFRC wall panels is expense. The making of GFRC wall panels is a labor intensive process wherein concrete and glass fibers are sprayed in a form. In addition to high labor costs associated with GFRC fabrication, the material cost of the glass fibers adds significantly to the overall cost of a GFRC wall panel.
Another relatively lightweight wall panel system that is being used today is commonly known as “slender wall.” Slender wall prefabricated wall panels typically include a relatively thin steel-reinforced concrete slab with structural steel framing rigidly attached to one side of the slab. A disadvantage of the slender wall system is that it requires the concrete supplier to fabricate the metal frame backup system, which requires a significant amount of design and fabrication time. Another disadvantage is that the inside face of the metal frame must be in near perfect alignment for proper drywall attachment.